His guarded response to prisoner abuse cost him an opportunity to show leadership, critics say. Backers defend actions as justifiably measured.

In a speech nine days ago in Missouri, Sen. John F. Kerry laid out his case against President Bush on Iraq, saying the administration had left America dangerously isolated from the rest of the world.

But the presumed Democratic nominee did not mention the brewing international furor over the mistreatment of Iraqi detainees by U.S. soldiers at the Abu Ghraib prison, an issue that seemed ready-made to bolster his critique.

The Massachusetts senator's initially guarded response to the scandal underscores the delicate balancing act he faces in calibrating his reactions to developments in Iraq.

Though setbacks for the U.S. may strengthen Kerry's assertion that Bush is mishandling the conflict, any criticism he makes of the military effort threatens to undermine the Democrat's portrayal of himself as a national security hawk who unflinchingly backs U.S. troops, political experts said. At the same time, some analysts said, Kerry has not made full use of the opportunity to present himself as a viable alternative.

The difficult political terrain led to the candidate's early reticence in addressing a surge of violence that gripped Iraq last month, and most recently, the graphic images of prisoners being humiliated and tortured by their U.S. guards.

"It's a tightrope walk for John Kerry," said Roy Neel, a longtime aide to Al Gore who took the helm of former Vermont Gov. Howard Dean's presidential bid in its final months. "This has been a problem he's had to deal with for some time. The danger is that if he's too strident on the prisoner issue, it could backfire as demagoguery."

Two days after CBS broadcast photos of U.S. troops abusing Iraqi prisoners, Kerry released a statement saying he was "disturbed and troubled" by the images. But he did not speak out publicly about the abuses for a week.

Since then, as the White House has come under increasing fire for its handling of the crisis and some legislators have called for the resignation of Defense Secretary Donald H. Rumsfeld, Kerry has sharpened his rhetoric.

On Friday, he made his most forceful statement yet about the prisoners' mistreatment, calling on the president to take full responsibility for the military's handling of the abuses. Meanwhile, his campaign circulated an e-mail petition to supporters demanding that Rumsfeld resign.

But some analysts said that by responding so slowly, Kerry lost a valuable opportunity to emphasize how he would handle the situation in Iraq.

"If he's going to win this election, he needs to be articulating a different vision for Iraq," said Joseph Tuman, who teaches political communication at San Francisco State University.

"This issue is one more example of how he's not showing how he's different than Bush."

But Kerry aides said there was no advantage for the candidate to address the issue before Bush did.

"Why would we want to jump on a story like this and politicize it before the facts were out?" asked spokeswoman Stephanie Cutter. "This is a pretty serious case. We felt that this is something that the administration should answer first."

Some allies concurred, praising Kerry's measured response in the early days of the scandal as politically astute.

"The Iraqi question is such a double-edged sword," said Pennsylvania Gov. Ed Rendell, a former chairman of the Democratic National Committee. "The worse things become, there is a tendency of the American people to want to rally around the commander-in-chief."

Indeed, Republicans warned that efforts to politically capitalize on the scandal would backfire on the Democratic presidential candidate.

"I don't see how Kerry could criticize the president on this," said Charlie Black, a veteran Republican strategist and informal advisor to the Bush-Cheney campaign. "If he did, he'd, in effect, be criticizing the U.S. military, which would be a dangerous position for him to take."

Throughout the campaign, Kerry has struggled to deliver a clear message about his stance on Iraq. After voting in October 2002 to authorize military action there, he distanced himself from the administration's handling of the invasion while Dean rode a wave of antiwar sentiment in the months before the Democratic primaries and caucuses.

Meanwhile, Kerry has sought to use his position as a decorated Vietnam veteran to reinforce his national security credentials, only to be confronted by Republicans who have challenged the seriousness of his wounds and his behavior as an antiwar activist after he left the Navy.

In recent weeks, the senator has tried to simultaneously emphasize his support for the troops and distinguish his approach on Iraq from that of the administration.